1. Field of the Invention
This invention relates to a method of automatically correcting the density of a subtraction image so that the same appropriate background density is always obtained in a subtraction processing of radiation images, and an apparatus for carrying out the method. This invention particularly relates to a method of automatically correcting the density of a subtraction image so that the same appropriate background density is always obtained in a subtraction processing of radiation images wherein stimulable phosphor sheets are used, and an apparatus for carrying out the method.
2. Description of the Prior Art
Conventionally, a digital subtraction processing method is used for processing radiation images. In the method, two radiation images recorded under conditions different from each other are photoelectrically read out to obtain digital image signals, which are then subjected to a subtraction processing with respect to the corresponding picture elements of the images, thereby to obtain a difference signal for forming an image of a specific structure contained in the radiation images. The method makes it possible to reproduce a radiation image of only the specific structure by use of the signal thus obtained.
Basically, the subtraction processing is classified into the so-called temporal (time difference) subtraction processing method and the so-called energy subtraction processing method. In the former method, the image of a specific structure is extracted by subtracting the image signal of a radiation image obtained without injection of contrast media from the image signal of a radiation image in which the image of the specific structure is enhanced by the injection of contrast media. In the latter method, an object is exposed to radiations having energy distributions different from each other to obtain two radiation images containing image information at least a part of which is different from each other. Then, the image signals of the two radiation images are weighted appropriately, and subjected to subtraction to extract the image of the specific structure in the object.
Since the subtraction processing is extremely effective for diagnostic purposes in image processings for medical X-ray photographs, it has recently attracted much attention, and research has continued to develop improved methods by use of electronic technology. The processing technique is particularly called digital subtraction processing method.
A novel digital subtraction processing method has been proposed, for example, in U.S. patent application Ser. No. 477,542. The method comprises the steps of (i) using two or more stimulable phosphor sheets exhibiting an extremely wide latitude of exposure to a radiation, (ii) exposing the stimulable phosphor sheets to the radiation passing through the same object with and without injection of contrast media to have radiation images of the object stored in the stimulable phosphor sheets, a part of image information being different between the radiation images due to injection of contrast media, (iii) detecting the radiation images by scanning with stimulating rays to obtain digital image signals, and (iv) conducting a digital subtraction processing by use of the digital image signals. The stimulable phosphor sheets comprise a stimulable phosphor which is able to store a part of radiation energy when exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, and then emit light in proportion to the stored energy of the radiation when exposed to stimulating rays such as visible light, as disclosed for example in U.S. Pat. No. 4,258,264. The stimulable phosphor sheets exhibit an extremely wide latitude of exposure and a markedly high resolving power. Therefore, when the digital subtraction processing is conducted by use of the radiation images stored in the stimulable phosphor sheets, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy.
When a subtraction image is formed on a photographic film or the like by use of the difference signal obtained by the aforesaid temporal subtraction processing, the image density at the portion outside of the specific structure injected with contrast media, i.e. at the background, should be always the same. However, in many cases, the background density is different between subtraction images. This is because the intensity of the radiation to which the stimulable phosphor sheets are actually exposed fluctuates slightly even if the radiation intensity is adjusted to the same value at the radiation image recording, and because the sensitivity of the stimulable phosphor sheets also fluctuates. When the background density is not the same, it is not always possible to correctly diagnose, for example, by comparing many subtraction images.